Strategist Tom Lee is known for perennially serving as one of the biggest bulls on Wall Street, but when asked about the month ahead, he's striking a markedly cautious tone.

"August scares us," the co-founder of Fundstrat Global Advisors said Monday on CNBC's "Trading Nation." He pointed to two troubling stats in particular.

Lee observes that going back to 2009, the S&P 500 has fallen an average of 6 percent during the month of August — a stat that jibes well with the "sell in May and go away" line of thinking.

This adds up to a "pretty scary" outlook for next month, Lee said. However, after an August slip, Lee expects a fall rip.

Or, as the strategist memorably put it in a recent note to clients: "sell the beach, buy the teach."

More specifically, Lee says that once the S&P falls to 2,100, "we think you should buy it," in anticipation of a substantial rally to Lee's 2,325 year-end price target. It closed Monday at 2,168.48.

Indeed, the long-term minded "shouldn't do anything, because a 2 to 3 percent sell-off isn't enough to warrant a big shift in a portfolio. But 2 to 3 percent for an active manager is relative performance."

Further, the odds of a slip could be even greater this year, given that "the bond market has become a lot more volatile than equities, and whenever this happens, 68 percent of the time, the stock market falls in the following month."Here is a daily chart of BONDS....man, Oh, Man, what a rollercoster ride ey? You can see the rising support line has been touched a few times but now. Hmmmmm. So its safe to say we are at a make or break area.